U.S. patent number 6,477,326 [Application Number 09/652,524] was granted by the patent office on 2002-11-05 for dual band framing reconnaissance camera.
This patent grant is currently assigned to Recon/Optical, Inc.. Invention is credited to Allie M. Baker, Stephen R. Beran, John Jones, Andrew J. Partynski, Stephan Wyatt.
United States Patent |
6,477,326 |
Partynski , et al. |
November 5, 2002 |
Dual band framing reconnaissance camera
Abstract
A framing aerial reconnaissance camera is described which has a
Cassegrain optical system forming an objective lens that directs
radiation to a spectrum-dividing prism. The prism directs radiation
in the visible portion of the electromagnetic spectrum into a first
optical path having a two-dimensional image-recording medium, such
as a framing CCD array. Radiation in the infrared (IR) band of the
spectrum is directed to a second optical path, which has a
two-dimensional framing IR-sensitive image-recording medium. The
entire camera can be either rotated about the aircraft roll axis in
a continuous fashion or stepped in a series of steps to generate
frames of imagery, providing panoramic coverage of the scene across
the line of flight in two bands of the spectrum simultaneously.
Inventors: |
Partynski; Andrew J. (Crystal
Lake, IL), Beran; Stephen R. (Mt. Prospect, IL), Baker;
Allie M. (Cypress, CA), Jones; John (Arkadelphia,
AR), Wyatt; Stephan (Crystal Lake, IL) |
Assignee: |
Recon/Optical, Inc.
(Barrington, IL)
|
Family
ID: |
24617140 |
Appl.
No.: |
09/652,524 |
Filed: |
August 31, 2000 |
Current U.S.
Class: |
396/7; 348/146;
396/8 |
Current CPC
Class: |
G02B
17/0852 (20130101); G02B 7/18 (20130101); G01C
11/025 (20130101); G02B 17/0808 (20130101); G02B
17/0896 (20130101); H04N 5/2251 (20130101) |
Current International
Class: |
G01C
11/02 (20060101); G01C 11/00 (20060101); G03B
039/00 (); H04N 007/18 () |
Field of
Search: |
;396/7,8,12,13
;348/144,145,146,147 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Integration of the KS-147A LOROP into RF--5E, Robert L. Walke, F-5
Airborne Equipment Design, Jeffrey P. Duda, EO and Recce Systems,
Northrop Corporation, SPIE vol. 561, Airborne Reconnaissance 1X,
pp. 26-33, Aug. 1985. .
The KS-1476A LOROP Camera System, Obert Ostrem and John G. Hughes,
Recon/Optical, Inc., SPIE vol. 561, Airborne Reconnaissance 1X, pp.
18-25, Aug. 1985. .
Image Stabilization Techniques for Long Range Reconnaissance,
Camera, George R. Lewis, Recon/Optical, Inc., pp. 1-6, Jul.
1980..
|
Primary Examiner: Adams; Russell
Assistant Examiner: Koval; Melissa
Attorney, Agent or Firm: McDonnell Boehnen Hulbert &
Berghoff
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is related to the following patent applications
filed on the same date as this application, the contents of each of
which is incorporated by reference herein:
Andrew J. Partynski et al., METHOD FOR FRAMING RECONNAISSANCE WITH
ROLL MOTION COMPENSATION, Ser. No. 09/654,031;
Stephen R. Beran et al., METHOD OF FORWARD MOTION COMPENSATION IN
AN AERIAL RECONNAISSANCE CAMERA, Ser. No. 09/652,965;
Stephen R. Beran et al., CASSEGRAIN OPTICAL SYSTEM FOR FRAMING
AERIAL RECONNAISSANCE CAMERA, Ser. No. 09/652,529.
Claims
We claim:
1. A dual band framing aerial reconnaissance camera for
installation in an aerial reconnaissance vehicle, comprising; (a)
an optical system incorporated into a camera housing, comprising:
(1) an objective optical subassembly for receiving incident
radiation from a scene external of said vehicle; (2) a spectrum
dividing element receiving radiation from said objective optical
subassembly, said element directing radiation in a first band of
the electromagnetic spectrum into a first optical path and
directing radiation in a second band of the electromagnetic
spectrum into a second optical path different from said first
optical path; (3) a first two-dimensional image recording medium in
said first optical path for generating frames of imagery in said
first band of the electromagnetic spectrum; and (4) a second
two-dimensional image recording medium in said second optical path
for generating frames of imagery in said second band of the
electromagnetic spectrum; (b) a first motor system coupled to said
camera housing rotating said camera about a first axis, said camera
housing installed in said aerial reconnaissance vehicle such that
said first axis of rotation is parallel to the roll axis of said
aerial reconnaissance vehicle, wherein said image recording media
are exposed to said scene to generate frames of imagery as said
first motor system rotates said camera in a continuous fashion
about said first axis, said first and second image recording media
having a means for compensating for image motion due to said
rotation of said camera; and (c) a second motor system coupled to
said objective optical subassembly, said second motor system
rotating said objective optical subassembly about a second axis in
the direction of forward motion of said vehicle to compensate for
forward motion of said aerial reconnaissance vehicle.
2. The camera of claim 1, wherein said first and second image
recording media comprise two dimensional area array electro-optical
detectors.
3. The camera of claim 2, wherein one of said electro-optical
detectors is sensitive to radiation in the ultraviolet (UV) portion
of the electromagnetic spectrum and wherein the other of said
electro-optical detectors is sensitive to radiation in the infrared
portion of the electromagnetic spectrum.
4. The camera of claim 3, wherein the detector sensitive to
radiation in the infrared portion of the electromagnetic spectrum
is sensitive to radiation having a wavelength of between 1.0 to 2.5
microns.
5. The camera as claimed in claim 2, wherein said electro-optical
detectors comprise an array of pixel elements arranged in a
plurality of rows and columns, and wherein said means for
compensation for roll motion of said camera housing comprises
electronic circuitry for transferring pixel information in said
electro-optical detectors from row to adjacent row at a pixel
information transfer rate substantially equal to the rate of image
motion in the plane of said electro-optical detectors due to roll
of said camera housing.
6. The camera as claimed in claim 5, wherein said camera further
comprises a camera control computer calculating said pixel
information transfer rate from system inputs comprising f, the
focal length of said optical system, and .omega., the rate of
rotation of said camera housing about said roll axis.
7. The camera as claimed in claim 1, wherein at least one of said
image-recording media comprises photosensitive film.
8. The camera as claimed in claim 7, wherein said means for
compensating for roll motion of said camera housing comprises a
mechanism for moving said film at a rate substantially equal to the
rate of image motion in the plane of said film due to roll of said
camera housing.
9. The camera as claimed in claim 8, wherein said camera further
comprises a camera control computer calculating said rate of
movement of said film from system inputs comprising f, the focal
length of said optical system, and .omega., the rate of rotation of
said camera housing about said roll axis.
10. The camera as claimed in claim 1, wherein said objective
optical subassembly comprises a catoptric Cassegrain optical system
which forms an image at a Cassegrain image plane, and wherein said
catoptric Cassegrain optical system comprises a primary mirror, a
secondary mirror rigidly coupled to said primary mirror, and a flat
azimuth mirror located in the optical path between the secondary
mirror and the Cassegrain image plane.
11. The camera as claimed in claim 10, wherein said second motor
system comprises a Cassegrain motor coupled to said primary mirror,
said secondary mirror and said azimuth mirror, and wherein to
compensate for forward motion of said vehicle said Cassegrain motor
rotates said primary and secondary mirrors in the flight direction
at a rate equal to V/R where V is the velocity of aerial
reconnaissance vehicle and R is either the range to the scene of
interest or an approximation of said range, and rotates said
azimuth mirror at rate equal to 1/2 (V/R) in the same direction as
the rotation of said primary and secondary mirrors due to said
Cassegrain motor.
12. The camera as claimed in claim 11, wherein the value of R is
derived from the height of said vehicle above the earth and a
camera depression angle below a horizontal reference frame.
13. The camera as claimed in claim 11, wherein the value of R is
derived using a range finder on board the vehicle.
14. The camera as claimed in claim 11, wherein the value of R is
derived from a global positioning system.
15. The camera as claimed in claim 11, wherein the value of R is
derived from processing successive frames of imagery from at least
one of said image-recording media.
16. The camera of claim 11, wherein said secondary mirror is
centrally located in the entrance aperture of said catoptric
Cassegrain optical system.
17. The camera as claimed in claim 1, wherein said optical system
has an overall focal length of between 50 and 100 inches and a
f/number of between 4.0 and 8.0.
18. The camera as claimed in claim 1, further comprising a camera
control computer operative of said image recording media and said
first and second motor systems to generate a series of overlapping
frames of imagery across the line of flight of said vehicle as said
camera housing is rotated about said first axis, and wherein each
of said overlapping frames of imagery is recorded in two different
portions of said electromagnetic spectrum by said first and second
image recording media.
19. The camera as claimed in claim 18, wherein said camera further
comprises a spot mode of operation, said camera control computer in
said spot mode of operation operative of said first and second
motor systems to orient said Cassegrain optical system at a
selected camera depression angle below a horizontal reference plane
and fore/aft azimuth angle, and operative of said first and second
image recording media to generate first and second frames of
imagery of said scene at said camera depression angle and fore/aft
azimuth angle.
20. The camera as claimed in claim 19, wherein said objective
optical subassembly comprises a Cassegrain optical system forms an
image at a Cassegrain image plane, and wherein said Cassegrain
optical system comprises a primary mirror, a secondary mirror
rigidly coupled to said primary mirror, and a flat azimuth mirror
located in the optical path between the secondary mirror and the
Cassegrain image plane.
21. The camera as claimed in claim 20, wherein said second motor
system comprises a Cassegrain motor coupled to said primary mirror
and said azimuth mirror, and wherein to compensate for forward
motion of said vehicle said Cassegrain motor rotates said primary
and secondary mirrors in the flight direction at a rate equal to
V/R where V is the velocity of aerial reconnaissance vehicle and R
is either the range to the scene of interest or an approximation of
said range, and wherein said azimuth mirror is rotated at rate
equal to 1/2 (V/R) in the same direction as the rotation of said
primary and secondary mirrors due to said Cassegrain motor.
22. The camera as claimed in claim 21, wherein the value of R is
derived from the height of said vehicle above the earth and a
camera depression angle below a horizontal reference frame.
23. The camera as claimed in claim 21, wherein the value of R is
derived using a range finder on board the vehicle.
24. The camera as claimed in claim 21, wherein the value of R is
derived from a global positioning system.
25. The camera as claimed in claim 21, wherein the value of R is
derived from a processing of successive images generated by at
least one of said image-recording media.
26. The camera as claimed in claim 20 wherein said secondary mirror
is centrally located in the entrance aperture of said Cassegrain
optical system.
27. The camera of claim 2, wherein one of said electro-optical
detectors is sensitive to radiation in the visible portion of the
electromagnetic spectrum and wherein the other of said
electro-optical detectors is sensitive to radiation in the infrared
portion of the electromagnetic spectrum.
28. The camera of claim 3, wherein said electro-optical detector
sensitive to radiation in the infrared portion of the
electromagnetic spectrum is sensitive to radiation having a
wavelength of between 3.0 and 5.0 microns.
29. The camera of claim 3, wherein said electro-optical detector
sensitive to radiation in the infrared portion of the
electromagnetic spectrum is sensitive to radiation having a
wavelength of between 8.0 and 14.0 microns.
30. The camera of claim 27 wherein said electro-optical detector
sensitive to radiation in the infrared portion of the
electromagnetic spectrum is sensitive to radiation having a
wavelength of between 1.0 and 2.5 microns.
31. The camera of claim 27, wherein said electro-optical detector
sensitive to radiation in the infrared portion of the
electromagnetic spectrum is sensitive to radiation having a
wavelength of between 3.0 and 5.0 microns.
32. The camera of claim 27, wherein said electro-optical detector
sensitive to radiation in the infrared portion of the
electromagnetic spectrum is sensitive to radiation having a
wavelength of between 8.0 and 14.0 microns.
33. A framing aerial reconnaissance camera for installation in an
aerial reconnaissance vehicle, comprising; (a) an optical system
incorporated into a camera housing, comprising: (1) an objective
optical subassembly receiving incident radiation from a scene
external of said vehicle; (2) an optical channel receiving
radiation from said objective optical subassembly, and (3) a
two-dimensional image recording medium in said optical channel for
generating frames of imagery in a band of the electromagnetic
spectrum, said optical channel including one or more optical
elements focusing radiation from said scene on said two-dimensional
image recording medium; (b) a first motor system coupled to said
camera housing rotating said camera about a first axis, said camera
housing installed in said aerial reconnaissance vehicle such that
said first axis of rotation is parallel to the roll axis of said
aerial reconnaissance vehicle, wherein said image recording medium
is exposed to said scene to generate frames of imagery as said
first motor system rotates said camera in a continuous fashion
about said first axis, said image recording medium having a means
for compensating for image motion due to said rotation of said
camera housing; and (c) a second motor system coupled to said
objective optical subassembly, said second motor system rotating
said objective optical subassembly about a second axis in the
direction of forward motion of said vehicle about an axis to
compensate for forward motion of said aerial reconnaissance
vehicle.
34. The camera of claim 33, wherein said image recording medium
comprises a two-dimensional area array electro-optical
detector.
35. The camera of claim 34, wherein said electro-optical detector
is sensitive to radiation in the ultraviolet portion of the
electromagnetic spectrum.
36. The camera of claim 34, wherein said electro-optical detector
is sensitive to radiation in the infrared portion of the
electromagnetic spectrum.
37. The camera of claim 36, wherein said electro-optical detector
is sensitive to radiation having a wavelength of between 1.0 to 2.5
microns.
38. The camera as claimed in claim 34, wherein electro-optical
detector comprises an array of pixel elements arranged in a
plurality of rows and columns, and wherein said means for
compensation for roll motion of said camera housing comprises
electronic circuitry for transferring pixel information in said
electro-optical detector from row to adjacent row at a pixel
information transfer rate substantially equal to the rate of image
motion in the plane of said electro-optical detector due to roll of
said camera housing.
39. The camera as claimed in claim 38, wherein said camera further
comprises a camera control computer calculating said pixel
information transfer rate from system inputs comprising f, the
focal length of said optical system, and .omega., the rate of
rotation of said camera housing about said roll axis.
40. The camera as claimed in claim 33, wherein said image recording
medium comprises photosensitive film.
41. The camera as claimed in claim 40, wherein said means for
compensating for roll motion of said camera housing comprises a
mechanism for moving said film at a rate substantially equal to the
rate of image motion in the plane of said film due to roll of said
camera housing.
42. The camera as claimed in claim 41, wherein said camera further
comprises a camera control computer calculating said rate of
movement of said film from system inputs comprising f, the focal
length of said optical system, and .omega., the rate of rotation of
said camera housing about said roll axis.
43. The camera as claimed in claim 33, wherein said optical has an
overall focal length of between 50 and 100 inches and a f/number of
between 4.0 and 8.0.
44. The camera as claimed in claim 33, further comprising a camera
control computer operative of said image recording medium and said
first and second motor systems to generate a series of overlapping
frames of imagery across the line of flight of said vehicle as said
camera housing is rotated about said first axis.
45. The camera as claimed in claim 44, wherein said camera further
comprises a spot mode of operation, said camera control computer in
said spot mode of operation operative of said first motor systems
to orient said objective optical subassembly at a selected camera
depression angle about the roll axis of said vehicle and fore/aft
azimuth angle, and operative of said first and second image
recording media to generate first and second frames of imagery of
said scene at said camera depression angle and fore/aft azimuth
angle.
46. The camera of claim 34, wherein said electro-optical detector
is sensitive to radiation in the visible portion of the
electromagnetic spectrum.
47. The camera of claim 36, wherein said electro-optical detector
is sensitive to radiation having a wavelength of between 3.0 and
5.0 microns.
48. The camera of claim 36, wherein said electro-optical detector
is sensitive to radiation having a wavelength of between 8.0 and
14.0 microns.
49. A framing aerial reconnaissance camera for installation in an
aerial reconnaissance vehicle, comprising; (a) an optical system
incorporated into a camera housing, comprising: (1) a Cassegrain
optical system for receiving incident radiation from a scene
external of said vehicle; (2) an optical channel receiving
radiation from said Cassegrain optical system, and (3) a
two-dimensional electro-optical detector in said optical channel
for generating frames of imagery in a band of the electromagnetic
spectrum, said optical channel including one or more optical
elements focusing radiation from said scene on said two-dimensional
electro-optical detector; (b) a first motor system coupled to said
camera housing rotating said camera about a first axis, said camera
housing installed in said aerial reconnaissance vehicle such that
said first axis of rotation is parallel to the roll axis of said
aerial reconnaissance vehicle, wherein said detector is exposed to
said scene to generate frames of imagery as said first motor system
rotates said camera in a continuous fashion about said first axis,
and (c) a second motor system coupled to said Cassegrain optical
system, said second motor system rotating said Cassegrain optical
system about a second axis in the direction of forward motion of
said vehicle about an axis to compensate for forward motion of said
aerial reconnaissance vehicle, wherein said electro-optical
detector comprises an array of pixel elements arranged in a
plurality of rows and columns, and wherein said detector further
comprises electronic circuitry coupled to said array transferring
pixel information in array from row to adjacent row at a pixel
information transfer rate substantially equal to the rate of image
motion in the plane of said electro-optical detectors due to roll
of said camera; said electronic circuitry and said second motor
system operative to achieve roll motion compensation and forward
motion compensation simultaneously in said array to thereby enable
high resolution images to be obtained from said electro-optical
detector.
50. The camera as claimed in claim 49, wherein said camera further
comprises a camera control computer calculating said pixel
information transfer rate from system inputs comprising f, the
focal length of said optical system, and so, the rate of rotation
of said camera housing about said roll axis.
51. A method of generating frames of imagery of a scene of interest
with an aerial reconnaissance camera in two different bands of the
electromagnetic spectrum simultaneously, comprising the steps of:
providing two photosensitive electro-optical detectors in said
camera, each of said detectors comprising an array of pixel
elements arranged in a plurality of rows and columns; rotating said
camera in a continuous fashion about a roll axis either coincident
with or parallel to a roll axis of an aerial reconnaissance vehicle
carrying said camera; while rotating said camera, simultaneously
exposing said electro-optical detectors to a scene of interest in a
series of exposures; while rotating said camera and while exposing
said electro-optical detectors to said scene, rotating an optical
system providing an objective lens for said camera in the direction
of forward motion of said vehicle at a predetermined rate to cancel
out image motion due to forward motion of said vehicle; and while
said electro-optical detectors are being exposed to said scene,
moving pixel information in said arrays at a rate and in a
direction substantially equal to the rate of image motion due to
rotation of said camera about said roll axis to thereby preserve
resolution of an image generated by said detectors.
Description
BACKGROUND OF THE INVENTION
A. Field of the Invention
This invention relates generally to the field of aerial
reconnaissance photography and camera systems used for such
photography. More particularly, in a principal aspect the invention
relates to a reconnaissance camera that generates frames of imagery
of terrain in different portions or bands of the electromagnetic
spectrum simultaneously.
The invention also relates to a novel method by which a camera
compensates for image motion due to both camera rotation and
forward motion of the aircraft in which the camera is installed.
Such image motion compensation allows for high-resolution images to
be obtained from the camera system.
B. Description of Related Art
Long Range Oblique Photography (LOROP) cameras have been developed
as a result of the need to obtain clear, high resolution pictures
from longer ranges, typically from 10 to 50 nautical miles from the
terrain of interest. The advent of LOROP cameras was an outgrowth
of development of weapons technology, which could engage
reconnaissance aircraft at ever-increasing distances, and
geopolitical boundaries that became more and more difficult to
encroach upon.
With the advent of LOROP cameras came the operational intricacies
of using very sensitive and high performance instruments in a
fashion that yielded the intelligence, i.e., image resolution,
required of them. These operational issues were hostage to the
technological limitations of the day. Initially, all cameras were
film. Film LOROP cameras have been operated both as panoramic
scanning (line scan) and framing cameras. Panoramic scan cameras
collect an image with a smooth rolling motion of the camera while
exposing film by pulling it passed a slit. The advantage of this
approach was ease of implementation of the scanning mechanism. The
disadvantage is that each line of exposed imagery was taken from a
different perspective, hence the scanning system inherently was
prone to creating geometrically and geospatially distorted
images.
Subsequently, LOROP film framing cameras were employed. These
cameras captured a frame of imagery by rapidly moving a slit across
the film for exposure. The cameras utilized a scan head mirror
assembly that could be moved in order to take successive frames of
imagery at a selected depression angle relative to the horizon,
depending on the target location.
Later, electro-optical line scan cameras entered the market as a
filmless solution. Instead of film, the cameras used a solid state
linear line scan charge coupled device (CCD) as a detector. These
cameras used a scan mirror or the motion of the ground below the
aircraft to scan the image across the line of photosensitive
detectors that made up the CCD to form a frame, line by line.
Again, the disadvantage of this method was that imagery was
obtained from a different perspective as the aircraft moved,
resulting in geometrically and geospatially distorted images.
Step framing cameras were developed which take a full frame of
imagery at one time, then step the camera to a new angular
position, take the next frame of imagery (with some overlap between
the images to insure 100% coverage), step and generate a new frame
of imagery, and so on until the desired scene is covered. The
disadvantage of step framing cameras was that the stepping action
was very difficult to accomplish with the whole camera, therefore
it had to be broken into a scan head that performed the stepping
and an image de-rotation mechanism, both of which were tied
together by a synchronized drive system. The advantages of step
frame cameras as compared to line scanning cameras are higher
geometric fidelity and geo-spatial accuracy. Originally, full
framing cameras were all film.
The next revolutionary step in the art of LOROP and tactical aerial
reconnaissance cameras was the development of two-dimensional area
array electro-optical (E-O) detectors. This occurred several years
after the electro-optical linear arrays were first developed, and
required semiconductor processing technology to mature many more
years before such arrays were practical for reconnaissance use.
Recon/Optical, Inc., the assignee of the present invention, in the
early 1990's, introduced large area focal plane arrays to the
reconnaissance industry. One such array is described in U.S. Pat.
No. 5,155,597 to Andre G. Lareau et al., the contents of which are
incorporated by reference herein. Such cameras were the first large
area arrays to be used in tactical aircraft, as well as strategic
reconnaissance aircraft such as the high altitude SR-71 aircraft.
These large area arrays had the advantage of providing an image
from a single point in space giving excellent geometric fidelity.
Moreover, the high pixel count, and optimal pixel size, allowed
such cameras to produce imagery having outstanding image
resolution.
Furthermore, as described in the '597 Lareau et al. patent, it was
possible to perform forward motion compensation in side oblique,
forward oblique and nadir camera orientations electronically. U.S.
Pat. No. 5,668,593, also to Lareau et al., describes a step-frame
electro-optic camera system with electronic forward motion
compensation. U.S. Pat. No. 5,798,786, also to Lareau et al.,
describes a method for compensation for roll, pitch or yaw motions
of an aerial reconnaissance vehicle, in addition to forward motion
compensation, electronically in the focal plane of an E-O detector.
The '593 and '786 Lareau et al. patents are incorporated by
reference herein.
Framing E-O LOROP camera systems were a logical platform to host
the advanced detectors such as described in the Lareau et al. '597
patent. Electro-optical detectors, such as described in the Lareau
et al. '597 patent, are capable of being fabricated from selected
materials that can detect incident radiation in a variety of
portions of the electromagnetic spectrum, and not just the visible
spectrum. In particular, the advantages of large area framing can
be enhanced by providing imaging capability in the infrared (IR)
portion of the spectrum. A camera that generates frames of imagery
in two distinct portions of the electromagnetic spectrum
simultaneously is referred to herein as a "dual band framing
camera." The patent to Gilbert W. Willey, U.S. Pat. No. 5,841,574,
also assigned to Recon/Optical, Inc., describes a multi-spectral,
decentered aperture, catadioptric optical system particularly
suitable for a dual band line scanning camera system having two
linear electro-optical detectors, one for the visible or near IR
(.lambda.=0.5 to about 1.0 microns), and one for either the
mid-wavelength IR (.lambda.=about 3.0 to about 5.0 microns) or the
long-wavelength IR (.lambda.=about 8.0 to about 14.0 microns).
The technological capability of dual band framing LOROP cameras
promises performance heretofore unavailable anywhere. However, the
implementation of such a camera presents a number of difficulties
and technical challenges beyond those posed for prior art systems.
These challenges are optical, servo-mechanical and operational, and
are discussed in further detail below. The present invention
provides a dual band framing aerial reconnaissance camera system
that overcomes these challenges and difficulties to provide an
advanced, high resolution framing camera system that generates
imagery of a scene of interest at two different bands of the
electromagnetic spectrum.
SUMMARY OF THE INVENTION
A dual-band framing aerial reconnaissance camera for installation
in an aerial reconnaissance vehicle has been invented. The camera
includes an optical system incorporated into a camera housing. The
optical system comprises an objective optical subassembly that
receives incident radiation from a scene external of the vehicle.
Radiation from the scene is reflected from the objective optical
subassembly to a spectrum-dividing prism. The prism directs
radiation in a first band of the electromagnetic spectrum, such as
visible and near IR, into a first optical path and directs
radiation in a second band of the electromagnetic spectrum, such as
midwavelength IR or long wavelength IR, into a second optical path
different from the first optical path. The first optical path
includes suitable image forming and focusing lenses and a first
two-dimensional image-recording medium for generating frames of
imagery in the first band of the electromagnetic spectrum. The
second optical path also includes suitable image forming and
focusing lenses and a second two-dimensional image-recording medium
generating frames of imagery in the second band of the
electromagnetic spectrum.
The camera further includes a servo-mechanical subsystem. This
subsystem includes a first motor system coupled to the camera
housing that rotates the entire camera housing (including the
optical system as recited above) about a first axis. The camera
housing is installed in the aerial reconnaissance vehicle such that
this first axis of rotation is parallel to the roll axis of the
aerial reconnaissance vehicle (referred to herein for simplicity as
"the roll axis"). The image recording media are exposed to the
scene to generate frames of imagery as the first motor system
rotates the camera housing in a continuous fashion about the roll
axis. The first and second image recording media have a means for
compensating for image motion due to the rotation of the camera
housing. In an electro-optical embodiment of the image recording
media, the roll motion compensation means is preferably comprised
of electronic circuitry for clocking or transferring pixel
information through the electro-optical detectors at a uniform rate
substantially equal to the rate of image motion due to camera
rotation. A method of calculating the image motion rate, and thus
pixel information transfer rate, due to roll of the camera housing
is disclosed herein. If a film camera is used for the image
recording media, a mechanical system is used to move the film at a
rate substantially equal to the image motion rate.
The servo-mechanical subsystem also includes a second motor system
coupled to the objective optical subassembly. In the illustrated
embodiment, the objective optical subassembly comprises a catoptric
Cassegrain optical system. The second motor system rotates the
Cassegrain optical system about a second axis in the direction of
forward motion of the reconnaissance Vehicle to compensate for the
forward motion of the aerial reconnaissance vehicle. The action of
the first motor assembly to rotate the entire camera housing about
the roll axis occurs at the same time (i.e., simultaneously with)
the action of the second motor system to rotate the Cassegrain
optical system in the line of flight to accomplish forward motion
compensation. The net effect of the action of the Cassegrain motor
system and the roll motion compensation system is that the image of
the scene of interest is essentially frozen relative to the focal
plane of the image recording media while the media obtain the
frames of imagery, allowing high resolution images of the scene in
two different bands of the spectrum to be obtained simultaneously.
Furthermore, the rotation of the image scene caused by the roll
motion of the objective subassembly is simultaneously detrotated by
the roll motion of the rest of the camera, in view of the fact that
the entire camera assembly is rolled as a unit, thereby eliminating
the need for a separate derotation mechanism such as a pechan
prism. Other types of optical arrangements for the objective
optical subassembly are possible, but are less preferred. The
operation of the camera with the different type of objective
subassembly is the same.
In a preferred embodiment, the first and second image recording
media comprise two dimensional area array electro-optical
detectors. One may be manufactured from materials sensitive to
radiation in the visible and near-IR portion of the electromagnetic
spectrum, and in a preferred embodiment is a charge-coupled device
(CCD) detector of say 5,000.times.5,000 pixels. The other of the
electro-optical detectors is made from a material sensitive to
radiation in the infrared portion of the electromagnetic spectrum,
and may be a platinum silicide array of photo diode detectors or
other suitable type of electro-optical detector suitable for IR
detection. The reader is directed to U.S. Pat. No. 5,925,883 to
Woolaway, III, the contents of which are incorporated by reference
herein, for a description of an IR detector. The detector sensitive
to radiation in the infrared portion of the electromagnetic
spectrum is preferably sensitive to radiation having a wavelength
of between 1.0 and 2.0 microns (SWIR), 3.0 and 5.0 microns (MWIR),
or from about 8.0 to about 14.0 microns (LWIR). In either of the
embodiment of electro-optical detectors, they will typically
comprise an array of pixel elements arranged in a plurality of rows
and columns. The means for compensation for roll motion of the
camera housing comprises electronic circuitry for transferring
pixel information in the electro-optical detectors from row to
adjacent row at a pixel information transfer rate (uniform across
the array) substantially equal to the rate of image motion in the
plane of the electro-optical detectors due to roll of the camera
housing. The transfer of pixel information occurs while the pixel
elements are integrating charge representing scene information.
Thus, the roll motion compensation can be performed electronically
on-chip.
As a further possible embodiment, electro-optical detectors with
the capability for transferring pixel information in both row and
column directions independently, such as described in Lareau et
al., U.S. Pat. No. 5,798,786, could be used for the image recording
media. Forward motion compensation and roll motion compensation
could be performed on-chip in the detectors.
As noted above, the present invention required the solution to
several difficult technical challenges, including optical,
servo-mechanical and operational difficulties. For an
electro-optical framing LOROP camera to operate in at least two
discrete bands of the electromagnetic spectrum at the same time,
the optical challenge is to focus panchromatic energy (e.g. visible
through IR) on a focal plane detector with (1) good image quality
and satisfactory modulation transfer function, (2) while baffling
stray energy, (3) meeting space constraints, and (4) enabling the
use of a relatively large two-dimensional area array as a focal
plane detector to get an adequate field of view and resolution. In
accordance with one aspect of the invention, these optical
challenges were solved by a unique catoptric Cassegrain objective
optical subassembly incorporating an azimuth mirror and utilizing
separate field optics for each band of the spectrum, described in
more detail herein.
The catoptric Cassegrain type of objective optical subassembly does
not lend itself to the use of servo-mechanical systems developed
for prior art LOROP systems, particularly those used in prior art
step frame cameras (such as described in the Lareau et al. '593
patent). The prior art step frame cameras use a stepping mirror to
step across the line of flight and direct radiation onto the array,
and require a de-rotation mechanism, such as a Pechan prism, to
de-rotate the images. The standard solution of stepping the entire
LOROP camera system or even a large scan mirror assembly at the
operational frame rate are not acceptable alternatives for large
LOROP cameras, an in particular large dual band systems. In
particular, the applications of the present invention are flexible
enough to include both strategic and tactical aircraft, as well as
the new breed of aircraft being used by the military for
reconnaissance known as unmanned aerial vehicles (including low
observables). The diversity of these applications posed a power and
stability problem that prevents application of prior art solutions.
The task of stepping a 400 lb. camera mass two to four times a
second creates tremendous inertial loads as well as power spikes
that would be unacceptable. Even the inertia and associated
settling times of a stepped scan head assembly pose problems in
some applications.
This servo-mechanical situation required a unique inventive
solution, described in detail herein. The solution, as provided in
one aspect of the present invention, was to (1) rotate the entire
camera (including the entire optical system and the image recording
media) smoothly in a continuous fashion about an axis parallel to
the aircraft roll axis, similar to the pan-type movement, but
without the starts and stops used in a traditional step-frame
camera system, and (2) operating the camera as a framing camera
while the camera undergoes the smooth rotation. Frames of imagery
are thus taken while the camera smoothly rotates about the roll
axis at a constant angular velocity. In addition to this novel
"roll-framing" technique, the present invention also electronically
compensates for, i.e., stops, the image motion due to roll while
the camera is scanning in a smooth motion. Meanwhile, a novel
forward motion compensation technique is performed by the
Cassegrain optical subassembly to cancel out image motion effects
due to the forward motion of the aircraft. The result enables
exposures of the image recording media to the scene while
compensating for roll and forward motion, enabling high-resolution
images to be obtained.
The present invention thus solves the difficult optical,
servo-mechanical and operational problems and provides a dual band
framing electro-optical LOROP camera that delivers a performance
and technical capability that has never before been achieved. In
particular, it provides a system by which high-resolution frames of
imagery in two different portions of the electromagnetic spectrum
can be generated simultaneously. The inventive camera can be used
in a quasi-stepping mode, in which overlapping frames of imagery
are obtained across the line of flight. It can also be used in a
spot mode, in which the camera is oriented in a particular
direction to take an image of a target expected to be in the field
of view.
Many of the teachings of the present invention are particularly
applicable to a dual band electro-optical framing reconnaissance
camera, and such a camera is the preferred embodiment. However, as
explained below, some of the techniques and methods of the subject
camera system, such as the roll-framing operation and unique roll
and forward motion compensation techniques, are applicable to a
camera system that images terrain in only one portion of the
electromagnetic spectrum. Thus, in an alternative embodiment the
camera is basically as set forth as described above, except that
only a single detector is used and the spectrum-dividing prism and
second optical path are not needed. Furthermore, while a preferred
embodiment uses a two-dimensional electro-optical imaging array for
the detector in each of the bands of the electromagnetic spectrum,
the inventive camera system can be adapted to use film or other
types of detectors for the photosensitive recording medium. In the
film camera embodiment, roll motion compensation could be performed
by moving the film in a manner such that the film velocity
substantially matches the image velocity due to roll.
While the foregoing summary has described some of the highlights of
the inventive camera system, further details on these and other
features will be described in the following detailed description of
a presently preferred embodiment of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Presently preferred embodiments of the invention will be discussed
below in conjunction with the appended drawing figures, wherein
like reference numerals refer to like elements in the various
views, and wherein:
FIG. 1 is a perspective view of an aircraft flying over a terrain
of interest with a camera in accordance with the preferred
embodiment operating to generate frames of imagery of the terrain
in two bands of the electromagnetic spectrum simultaneously.
FIG. 2 is a schematic representation of the aircraft of FIG. 1
taking a series of 5 frames of images in a series of cycles while
flying past the terrain of interest;
FIGS. 2A and 2B are perspective view of the camera system of FIG.
1, shown isolated from the rest of the aircraft, and with
protective covers removed in order to better illustrate the
components of the camera;
FIG. 2C is a perspective view of the camera of FIGS. 2A and 2B,
with the protective covers installed, and showing the entrance
aperture for the catoptric Cassegrain optical system;
FIG. 3 is a top plan view of a presently preferred embodiment of
the dual band framing reconnaissance camera system of FIGS. 2A-2C,
with the covers removed;
FIG. 4 is a cross-sectional view of the camera system of FIG. 3,
taken along the lines 4--4 of FIG. 3;
FIG. 4A is a simplified ray diagram of the optical system of FIGS.
3 and 4;
FIGS. 4B and C are more detailed cross-sectional views of the
optical elements in the visible and MWIR paths of FIGS. 4 and
4A;
FIG. 5 is an end view of the camera system of FIGS. 3-4, shown from
the right-hand end of the camera housing and with the roll motor
and cover plate at that end removed in order to better illustrate
the other structures in the camera;
FIG. 6 is a perspective view of the assembly of the Cassegrain
subsystem, showing in better detail the structure that retains the
Cassegrain primary mirror and showing the secondary mirror, azimuth
mirror, Cassegrain motor assembly and azimuth 2-1 drive assembly in
greater detail. The primary mirror itself is removed from FIG. 6 in
order to better illustrate the components of the Cassegrain optical
system.
FIG. 7 is another perspective view of the Cassegrain primary mirror
retaining assembly of FIG. 6;
FIG. 8 is another perspective view of the Cassegrain primary mirror
retaining assembly as seen generally from the rear, shown partially
in section;
FIG. 9 is a top view of the Cassegrain optical system of FIG.
6;
FIG. 10 is a cross-sectional view of the Cassegrain optical system
of FIG. 6, taken along the line 10--10 of FIG. 9;
FIG. 11 is a detailed sectional view of the azimuth mirror 2-1
drive assembly that rotates the azimuth mirror at one half the rate
of rotation of the entire Cassegrain optical subsystem;
FIG. 12 is a detailed perspective view of one of the roll motor
assemblies of FIG. 2, showing the L shaped brackets that mount to
the stator of the motor and rigidly couple the roll motor to the
pod or aircraft;
FIG. 13 is an elevational view of the roll motor of FIG. 12;
FIG. 14 is a cross-sectional view of the roll motor of FIG. 14;
FIG. 15 is a detailed illustration of a portion of the roll motor
of FIG. 14;
FIG. 16A is a ray diagram of the visible path in the embodiment of
FIG. 4;
FIG. 16B is a ray diagram of the MWIR path in the embodiment of
FIG. 4;
FIG. 16C is a graph of the visible path diffraction modulation
transfer function;
FIG. 17 is a block diagram of the electronics for the camera system
of FIGS. 2-5;
FIG. 18 is schematic representation on an image recording medium in
the form of a two dimensional electro-optical array, showing the
image motion in the array due to the roll of the camera; and
FIG. 19 is another schematic representation of the array of FIG.
18, showing the electronic circuitry that controls the transfer of
pixel information in the array at the same velocity as the image in
order to provide roll motion compensation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Overview and Method of Operation
Referring now to FIG. 1, an aerial reconnaissance camera system 20
in accordance with a preferred embodiment of the invention is shown
installed in a reconnaissance aircraft 22 flying over a terrain of
interest 42 at an altitude H and with forward velocity V, moving in
a direction of flight FL. The aerial reconnaissance camera system
20 includes a camera 36, shown in greater detail in FIGS. 2-2C and
3-5, a camera control computer 34 and associated electronics
described in further detail in FIGS. 17 and 19. The camera control
computer receives certain navigational information from the
aircraft avionics system 24, including current aircraft velocity
and height data. Additional camera system inputs may come from a
console 28 in the cockpit, such as start and stop commands or
camera depression (roll angle) settings.
The aircraft body defines a roll axis R, a pitch axis PI and a yaw
axis Y passing through the center of gravity CG of the aircraft.
The camera 36 is shown orientated at a camera depression angle
.delta. relative to a bilateral plane BP that is horizontal during
level flight. In the illustrated embodiment, the line of sight LOS
of the camera 36 is nominally orthogonal to the roll axis in a side
oblique or nadir orientation.
The preferred embodiment of the subject camera system 20 operates
like a step-frame electro-optic (E-O) sensor, capable of taking a
sequence of overlapped frames in the cross-track, i.e., cross-line
of flight, direction. This is shown in FIG. 2. As the aircraft
flies by the terrain of interest, the camera is rotated about the
roll axis in a continuous fashion (i.e., without starts and stops
between frames), with frames of imagery taken at different
depression (roll) angles, e.g., angles .delta.1, .delta.2,
.delta.3, .delta.4 and .delta.5, resulting in frames 1, 2, 3, 4 and
5. A nominal rate of rotation about the roll axis is used (based on
focal length, array frame size and framing rate, such as 8-10
degrees per second, but the roll rate is adjustable by the camera
control computer. When the fifth frame of imagery is obtained and
the camera rolled to its roll limit position (either pre-set or
commanded by the operator), the camera rotates back, i.e.,
retraces, to its initial roll position (.delta.1), and the second
cycle of frames of imagery is obtained (1A, 2A, 3A, 4A, 5A). The
process repeats for a third and subsequent cycles of operation.
The cross-track framing sequence 1, 2, 3, 4, 5; 1A, 2A, 3A, 4A, 5A;
etc. (which is V/H dependent) can be made in either spectrum
individually or in both spectrums simultaneously, dependent on the
time of day and the purpose of the reconnaissance mission. As noted
in FIG. 2, the roll action of the camera can encompass both sides
of nadir, for example with frames 1-4 obtained at one side of nadir
and frame 5 obtained at the other side of nadir. The camera can
also be used in a spot mode, in which the camera is rotated to a
particular depression angle and frames of imagery obtained of the
scene of interest. The number of frames per cycle of roll, N, can
thus vary from 1 to say 5 or 10 or until horizon to horizon
coverage is obtained.
FIG. 2A shows the camera 36 in a perspective view as seen from
below, with a set of protective cover plates removed in order to
better illustrate the structure of the camera. FIG. 2B is another
perspective view, shown from above, and FIG. 2C is a perspective
view of the camera 36 with the cover plates 33 installed, showing
the entrance pupil 35 for the camera. Referring now to FIGS. 3-5,
the camera 36 per se is shown in top, sectional and end views,
respectively. In the end view of FIG. 5, a rear support plate 41
and a roll motor 70A are removed in order to better illustrate the
rest of the camera 36.
As shown best in FIG. 3, the camera 36 mounts to the reconnaissance
pod or airframe of the aerial reconnaissance vehicle via four
mounting brackets 39, each connected to the pod or airframe via
passive shock isolation mounts in conventional fashion. The
mounting brackets 39 are bolted to the sides of the stator of the
roll motor assemblies 70A and 70B as shown in FIG. 12 and described
below. The entire camera cylinder comprising all the components
between the two support plates 41 and 41A can rotate relative to
the roll axis 37 while the stator of the roll motors 70A and 70B
and mounting brackets 39 remain in a fixed position relative to the
aerial reconnaissance vehicle.
The basic configuration of the camera 36 is a cylinder, as perhaps
best illustrated in FIG. 2C, which in the illustrated embodiment is
approximately 20 inches in diameter and 48 inches in length. The
camera 36 is installed in an aircraft reconnaissance pod via the
mounting brackets 39 such that the cylinder axis 37 is oriented
nominally parallel to the flight direction of the aircraft, i.e.,
the roll axis of the aircraft. The fore/aft orientation of the
camera can be either way. Additionally, the camera 36 can be
installed such that it is oriented perpendicular to the line of
flight.
A typical use of the camera is to take overlapping frames of images
in the cross-track direction as the aircraft flies over the scene
of interest as shown in FIG. 2, similar in concept to the step
frame operation described in the prior art patent of Lareau, et al.
U.S. Pat. No. 5,668,593 and earlier step frame film cameras.
However, the manner in which the camera achieves this result is
very different from that taught in the prior art. Whereas in the
Lareau '593 patent, a stepping mirror is rotated in discrete steps
to image the terrain, and forward motion compensation is performed
in the array itself electronically, in the preferred embodiment of
the present invention the entire camera 36 is rotated at a constant
angular velocity, and in a continuous fashion, about the roll axis
37. The roll rate is determined by the optical system focal length,
frame size, frame rate and the desired cross-track overlap
(typically 5%) between consecutive frames. Moreover, forward motion
compensation is achieved by means of rotation of the Cassegrain
optical system about an axis 75, as described below, not in the
array.
Referring to FIGS. 3 and 4, the camera includes an optical system
50 which is incorporated into (i.e., mounted to) a camera housing
or superstructure 52. The optical system 50 in the preferred
embodiment comprises a novel catoptric Cassegrain objective optical
subassembly 54 which receives incident radiation from a scene
external of the vehicle. FIG. 4A shows a simplified ray diagram for
the optical system 50. The Cassegrain objective optical subassembly
includes a primary mirror 80, a secondary mirror 82 and a flat
azimuth mirror 84. The secondary mirror 82 is centrally located in
the entrance aperture of the Cassegrain optical system. Radiation
from the scene is reflected from the Cassegrain objective
subassembly 54 to a spectrum-dividing prism 56. The prism 56
directs radiation in a first band of the electromagnetic spectrum,
such as visible and near IR, into a first optical path 58 and
directs radiation in a second band of the electromagnetic spectrum,
such as mid-wavelength IR or long wavelength IR, into a second
optical path 60 different from the first optical path. The first
optical path 58 includes suitable image forming and focusing lenses
62 and a first two-dimensional image recording medium 64 for
generating frames of imagery in the first band of the
electromagnetic spectrum. The second optical path 60 includes a
fold prism 61, suitable image forming and focusing lenses 66 and a
second two-dimensional image recording medium 68 which generates
frames of imagery in the second band of the electromagnetic
spectrum.
The camera further includes a novel servo-mechanical subsystem.
This subsystem includes a first motor system 70A and 70B coupled to
the camera housing 52 that rotates the camera housing 52 (including
the optical system 50 as recited above) about the roll axis 37. The
image recording media 64 and 68 are exposed to the scene to
generate frames of imagery as the first motor system 70A and 70B
rotates the camera housing 52 in a continuous fashion about the
roll axis 37. The first and second image recording media have a
means for compensating for image motion due to the rotation of the
camera housing. In an electro-optical embodiment of the image
recording media, the roll motion compensation means is preferably
comprised of electronic circuitry for clocking or transferring
pixel information through the electro-optical detectors at a
uniform rate substantially equal to the rate of image motion due to
camera rotation. A method of calculating the image motion rate, and
thus pixel information transfer rate, due to roll of the camera
housing is described below. If a film camera is used for the image
recording media, a mechanical system is used to move the film at a
rate substantially equal to the image motion rate. Film drive
mechanisms for moving film for purposes of motion compensation are
known in the art and can be adapted for a film framing camera for
purposes of roll motion compensation by persons skilled in the
art.
The servo-mechanical subsystem also includes a second motor system
74, shown best in FIGS. 3, 5 and 6, coupled to the front end of the
Cassegrain optical system 54. The second motor system 74 rotates
the Cassegrain objective subassembly 54, including the primary,
secondary and azimuth mirrors, about a second axis 75 in the
direction of forward motion of the reconnaissance vehicle in a
manner to compensate for forward motion of the aerial
reconnaissance vehicle. In the illustrated embodiment, the azimuth
mirror 84 is rotated about the axis 75 at one half the rate of
rotation of the Cassegrain primary and secondary mirrors 80 and 82
in the direction of forward motion. The action of the first motor
assembly 70A and 70B to rotate the entire camera housing about the
roll axis occurs at the same time (i.e., simultaneously with) the
action of the second motor system 74 to rotate the Cassegrain
optical system 80, 82 and 84 in the line of flight to accomplish
forward motion compensation. The net effect of the action of the
Cassegrain motor system 74 and the roll motion compensation
technique is that the image of the scene of interest is essentially
frozen relative to the focal plane of the image recording media
while the image recording media obtain the frames of imagery,
allowing high resolution images of the scene in two different bands
of the spectrum to be obtained simultaneously.
During operation, as the entire camera 36 rotates by action of the
roll motors 70A and 70B, exposure of the detectors 64 and 68 at the
two focal planes is made. In the illustrated embodiment, in the
visible spectrum path 58 the exposure is executed by means of a
mechanical focal plane shutter 88 which opens to allow incident
photons to impinge on a two-dimensional charge-coupled device E-O
detector array 64. In the MWIR path 60, exposure is executed by
electronic switching (on/off) of IR-sensitive photocells arranged
in a two-dimensional array 68, basically by dumping charge
accumulating prior to the initiation of exposure and then
accumulating and storing charge when the exposure period commences.
However, any method of exposure control will work with this
roll-framing camera.
When the initial exposure is complete, the data is read out from
the two focal plane detectors 64 and 68 and they are placed in
condition for a second exposure. The rotation of the entire camera
assembly about the roll axis 37 continues smoothly (that is,
without starting and stopping as for example found in a prior art
step frame camera system). When the next exposure is ready to be
taken, i.e., when depression angle .delta.2 of FIG. 2 has been
reached, the shutter is opened in the visible/near IR path;
similarly, in the MWIR path the charge dumping ceases and charge is
accumulated. The data is then read out of the two focal plane
sensors after the exposure period is over. Meanwhile, the rotation
of the entire camera system about the roll axis continues without
interruption and a third and subsequent exposure of both cameras is
taken if time permits. The process continues until the angular
limit of the framing cycle has been reached, at which time the roll
motors 70A and 70B retrace their angular rotation and return to
their original angular position. The process then repeats for a new
cycle of framing, as indicated in FIG. 2.
The camera system roll rate (the cylinder angular velocity),
.omega., is established as follows. First, determine the
cross-track field of view per frame, .omega., according to equation
(1):
where W=detector array size in the cross-track direction; and
f=lens focal length (i.e, the focal length of the overall aggregate
of optical components in the particular band of interest, e.g., the
visible band).
Then, the cylinder angular velocity .omega. is computed according
to equation (2):
where FR=system frame rate (frames per second) OL.sub.c =overlap
between consecutive cross-track frames (expressed as a
decimal).
Note that the cylinder angular velocity .omega. is independent of
the aircraft's velocity and height above the earth. Typical angular
rotations between the successive exposures of the array will be
less than 10 degrees.
Since the focal plane detectors are rotating about the roll axis
during the exposure period, the scene image is translating across
each of the detector arrays in the cross-track direction at a fixed
velocity v=f.omega.. The image motion due to camera roll is
constant and uniform across the array. To compensate for this image
motion, and thereby preserve resolution, this image motion is
synchronized with the velocity at which charge representing scene
information is transferred within the detector arrays, thus
eliminating relative motion between the image and the pixels
imaging the scene and thus eliminating the image smear that would
otherwise take place at the detector. In other words, pixel
information in the entire array is transferred in the direction of
image motion from row to adjacent row at a rate that substantially
matches the image velocity v.
At the end of the exposure period (typically 0.0005 to 0.020
seconds), the cylinder continues rotating to the next scene
position while the collected scene signals are read-out of the
detector array(s). Note that there is no rotational start and stop
between exposures, as found in prior art step frame camera systems,
thereby avoiding the servo loop settling times, load current
surges, and power spikes produced by mechanical stepping systems as
noted earlier.
In this "roll-framing" type of operation, the two focal plane
detectors 64 and 68 operate in the above manner, taking N
consecutive cross-track frames, N being dependent on the time
available or by the intended mode (maximum coverage, limited
coverage or spot mode) of operation. The result is a series of
frames of images similar to that produced with a step frame camera
system, as indicated in FIG. 2, each frame taken in two different
bands of the electromagnetic spectrum. In maximum coverage mode, N
is determined by the V/H ratio of the mission, the camera system
depression angle range and the framing rate, and N can be as many
as 10 frames/cycle (or more) in normal operation. At the end of the
cross-track cycle, the camera system or cylinder is rotated back
(reset) to the first frame angular position and the cycle repeats
until the intended in-flight direction coverage is achieved. The
camera can generate overlapping frames of imagery similar to that
shown in FIG. 1, where N in the illustrated example=5.
In spot mode, a one or two frames/cycle is executed, with the
camera aimed at a specific predetermined depression (roll) angle
and fore/aft azimuth angle where a target or specific interest is
expected to be. In this example, N will typically equal 1 or 2. The
cycle may repeat for as many times as needed.
As another mode of operation, the camera could be used in a
traditional step frame operation. In this mode, the camera would
rotate between successive angular positions, and the photosensitive
media would generate two-dimensional images of the terrain. If the
camera body rotation is stopped during scene exposure, forward
motion compensation could be performed in the photosensitive media,
such as described in the earlier Lareau et al. patents.
The preferred forward motion compensation method will now be
described with a little more specificity. As the exposures are made
at either of the two detectors 64 and 68, the aircraft is moving at
some known velocity. The forward motion of the aircraft is
neutralized in a novel way in the preferred embodiment. Whereas in
the prior art Lareau et al. '586 patent forward motion compensation
is performed on-chip in the array, the forward motion compensation
of the preferred embodiment is performed by rotation of the
Cassegrain objective subassembly, i.e, the Cassegrain primary and
secondary mirror assembly, in the flight direction at a rate=V/R
(in units of radians per second) where V is the aircraft velocity
and R is the range to the scene of interest. The value of R can be
derived from simple geometry from the known aircraft height and
camera depression angle (.delta..sub.i) and assuming the earth is
flat in the scene of interest, from a Global Positioning System on
board the aircraft, using an active range finder, or by computing
range from successive frames of imagery as described in the patent
of Lareau et al., U.S. Pat. No. 5,692,062, which is incorporated by
reference herein. As the Cassegrain primary and secondary mirrors
80 and 82, respectively, are rotated at the V/R rate in the
direction of flight, the flat azimuth mirror 84, located in the
optical path between the secondary reflector and the Cassegrain
image plane 86, is rotated at a rate equal to 1/2 (V/R) in the same
direction, thus "stopping" image motion due to aircraft forward
motion at the image plane. Thus, the rotating Cassegrain objective
lens and the half speed azimuth mirror provide the needed forward
motion compensation function.
As an alternative embodiment, the Cassegrain optical system could
remain fixed and both forward motion compensation and roll motion
compensation could be performed in the focal plane detector by
transferring pixel information in both row and column directions in
accordance with the principles of the Lareau et al. patent, U.S.
Pat. No. 5,798,786.
From the FIGS. 1-5 and 18 and the above discussion, it will be
appreciated that we have invented a method of generating frames of
imagery of a scene of interest with an aerial reconnaissance camera
in two different bands of the electromagnetic spectrum
simultaneously. The method includes the steps of: (a) providing two
photosensitive electro-optical detectors 64, 68 in the camera 36,
each of the detectors comprising an array of pixel elements
arranged in a plurality of rows and columns; (b) rotating the
camera 36 in a continuous fashion about a roll axis 37 either
coincident with or parallel to a roll axis R of an aerial
reconnaissance vehicle carrying the camera; (c) while rotating the
camera 36, simultaneously exposing the electro-optical detectors
64, 68 to a scene of interest in a series of exposures; (d) while
rotating the camera 36 and while exposing the electro-optical
detectors 64 and 68 to the scene, rotating an optical system 54
providing an objective lens for the camera in the direction of
forward motion of the vehicle at a predetermined rate to cancel out
image motion due to forward motion of the vehicle; and (e) while
the electro-optical detectors 64 and 68 are being exposed to the
scene, moving pixel information in the arrays at a rate and in a
direction substantially equal to the rate of image motion due to
rotation of the camera about the roll axis, to thereby preserve
resolution of images generated by the detectors.
Table 1 lists performance specifications for a presently preferred
dual band step frame camera system in accordance with the
invention.
TABLE 1 Focal Length & f/# Visible Channel 50.0 inches-f/4.0
(Options) 72.0 inches-f/5.8 84.0 inches-f/6.7 MWIR Channel 50.0
inches-f/4.0 Optical System Type: Cassegrain objective lens with
spectrum beam divider and individual visible channel and MWIR
channel relay lenses. Operating Spectrums = Visible Channel -0.50to
0.90 microns MWIR Channel -3.0 to 5.0 microns Entrance Pupil
Diameter: 12.5 inches, both channels, all focal lengths. Detectors:
Visible Channel: 5040 .times. 5040 pixels .010 mm .times. .010 mm
pixel pitch 50.4 mm .times. 50.4 mm array size 4.0 frames/sec max.
MWIR Channel: 2016 .times. 2016 pixels .025 mm .times. .025 pixel
pitch 50.4 mm .times. 50.4 mm array size 4.0 frames/sec max. MWIR
Channel: 2520 .times. 2520 pixels (future) .020 mm .times. .020 mm
pixel pitch 50.4 mm .times. 50.4 mm array size 4.0 frames/sec max.
FOV (per frame): VIS Channel: 2.27.degree. .times. 2.27.degree. (50
inch F.L.) 1.58.degree. .times. 1.58.degree. (72 inch F.L.)
1.35.degree. .times. 1.35.degree. (84 inch F.L.) MWIR Channel:
2.27.degree. .times. 2.27.degree. (50 inch F.L.) Frame Rates:
Variable, up to 4.0 fr/sec Both channels, aIl focal lengths. Pixel
IFOV: VIS Channel: 7.9 .times. 10.sup.-6 RAD (50 inch F.L.) 5.5
.times. 10.sup.-6 RAD (72 inch F.L.) 4.7 .times. 10.sup.-6 RAD (84
inch F.L.) MWIR Channel: 19.7 .times. 10.sup.-6 RAD (50 inch F.L.)
15.8 .times. 10.sup.-6 RAD (50 inch F.L.) (future) Ground
Resolvable Distance (GRD) (at range, perpendicular to the LOS). VIS
Channel: 3 ft @ 31 N mi. (NIIRS-5) (50") 3 ft @ 45 N mi.(NIIRS-5)
(72") 3 ft @ 52 N mi. (NIIRS-5) (84") MWIR Channel: 3 ft @ 12.5 N
mi. (NIIRS-5) (50") (Future) 3 ft @ 15.6 N mi. (NIIRS-5) (50")
Field of Regard: Horizon to Horizon, or as limited by vehicle
windows (5.degree. to 30.degree. depression below horizon (.delta.)
is typical). Scene coverage Variable cross-track. rate: Roll rate:
8.6.degree./sec-(50 inch focal length, 4 Fr/Sec.)
Preferred Dual Band Camera Detailed Mechanical and Servo-Mechanical
Description
With the above overall description in mind, attention is directed
primarily to FIGS. 2A, 2B, and 3-5. The more important mechanical
aspects of the camera will now be described. The optical system 50,
including the Cassegrain optical system, spectrum dividing prism
56, and optical components in the optical paths 58 and 60, are
rigidly mounted to a camera housing or superstructure 52. This
camera housing 52 takes the form of a pair of opposed, elongate
C-shaped frames extending transversely on opposite sides of the
roll axis substantially the entire length of the camera. The
C-shaped frame members 52 provide a structure in which to mount the
various optical and mechanical components of the camera, including
the end plates 41 and 41A.
The end plate 41 is bolted to the right hand end of the C-shaped
frames 52, as shown in FIG. 3. The rotor portion of the roll motor
70A is in turn bolted to the end plate 41, thereby coupling the
rotational portion of the roll motor 70A to the camera frame 52.
The stator portion of the roll motor 70A is fixedly coupled to the
aircraft frame or pod via two L-shaped brackets 39 and the
associated passive isolation mounts (conventional, not shown). The
left-hand end of the C-shaped frame 52 is similarly bolted to an
end plate 41A, and the rotor portion of the roll motor 70B is
bolted to the end plate 41A, with the stator portion bolted to two
L-shaped brackets 39. Two roll motors 70A and 70B are conventional
frameless DC torque motors, adapted to mount to the camera 36. Two
are used in the illustrated embodiment in order supply enough
torque to rotate the camera housing 52 and all the attached
components, but one motor may suffice if it is powerful enough. In
the illustrated embodiment, the roll motors 70A and 70B are
frameless DC torque motors, adapted to fit to the camera housing, a
task within the ability of persons skilled in the art. The roll
motors are described below in further detail in conjunction with
FIGS. 12-15.
FIG. 3 is a top view of the camera 36, looking towards to the back
side of the primary mirror 80. The Cassegrain objective lens
optical subassembly 54 includes a primary mirror cell 100 which
includes four mounting flanges 102 with bolt holes 104 for mounting
via bolts to the top flange 106 of the C-shaped frames 52. The
Cassegrain optical system is shown isolated in FIGS. 6-10. In FIGS.
6 and 7, the primary mirror is removed in order to better
illustrate the rest of the structure in the Cassegrain optical
system.
As is shown best in FIG. 3 and 6, a spider 120 consisting of eight
arms 122 extends between an inner primary mirror holding ring 110
and an azimuth mirror mounting plate 124 located at the center of
the primary mirror 80. The mounting plate 124 incorporates three
adjustment screws 126 for adjusting the tilt of the azimuth mirror
84. A fiber optic gyroscope 128 is also mounted to the plate 124
and is provided for purposes of inertial stiffness and
stabilization of the Cassegrain optical system 50. The secondary
mirror assembly 113 includes a set of three adjustment screws 126A
for adjusting and aligning the orientation of the secondary mirror
relative to the primary mirror.
The stator portion of the Cassegrain motor 74 is fixed with respect
to the primary mirror cell 100. The rotor portion of the motor 74
is mounted to an annular ring 111 shown in FIG. 10, which is
attached to the inner primary mirror holding ring 110. The
secondary mirror 82 is fixed with respect to the primary mirror by
means of three arms 112. Thus, the motor 74 rotates both the
primary, secondary and azimuth mirrors about axis 75 in the
direction of the line of flight in unison. The Cassegrain motor 74
is based on a DC direct drive motor adapted as required to the
Cassegrain primary mirror holding structure, again a task within
the ability of persons skilled in the art.
The rotation of the inner mirror holding ring 110 by the Cassegrain
motor 74 is reduced by a two-to-one reduction tape drive assembly
114, shown best in FIGS. 5, 67, 9 and 11. The tape drive assembly
114 rotates an azimuth mirror drive shaft 116 that extends from the
tape drive assembly 114 to the azimuth mirror 84. The azimuth tape
drive assembly 114 rotates the azimuth mirror drive shaft 116 and
thus the azimuth mirror 84 at one half of the rate of rotation of
the primary and secondary mirrors by the Cassegrain motor 74.
The tape drive assembly 114 includes a two-to-one drive housing
150, two-to-one drive couplings 152 and 154, a shaft locking
coupling 156, and a pair of stainless steel tapes 158 and 160, the
thickness of which is shown exaggerated in FIG. 11.
Referring to FIG. 12, the roll motor 70A is shown isolated from the
rest of the camera in a perspective view. FIG. 13 is an elevational
view of the motor as seen from the other side. FIG. 14 is a
cross-sectional view of the motor 70A. The roll motor 70B is
identical to the motor of FIGS. 12-14. Additional details
concerning the tape drive assembly 114 are conventional and
therefore omitted for the sake of brevity.
The motor 70A includes a trunnion 200, a journal 202 and a DC
frameless motor 204. The journal 202 bolts to the plate 41 (FIG. 3)
via six bolt holes 208. A set of apertures 210 is provided in the
face of the journal 202 to reduce weight. The sides of the trunnion
200 have opposed, parallel flat surfaces 212 with a series of
mounting holes for enabling the L-shaped mounting brackets 39 to
mount to the trunnion 200 in a plurality of different positions.
The motor 70A also includes an electronics module contained in a
housing 214. The module includes a power amplifier and associated
DC electronic components, which are conventional.
As shown in the cross-sectional view of FIG. 14, and the detail of
FIG. 15, the motor assembly 70A also includes an annular shim 220,
an annular bearing 222, a lock washer 224 and locking nut 226, a
trunnion sleeve 228, a bearing spacer 230, a bearing insert 232 and
a bearing adjustment plate 234. Additional mechanical features
shown in FIGS. 14 and 15 are not particularly important and
therefore are omitted from the present discussion.
Optical System Detailed Description
The optical system design of the subject camera is driven by the
need to illuminate a large focal plane image recording medium and
by space constraints, namely the total axial length and the total
diameter, which have to be accounted for in potential aircraft
installation applications. Thus, while the particular optical
design described herein is optimized for a given set of spatial
constraints, variation from the illustrated embodiment is
considered to be within the scope of the invention.
The optical system 50 of FIGS. 3 and 4 represents a 50-inch, F/4
optical system designed to operate over an extended spectral
region. The objective lens module consists of the Cassegrain
optical subsystem 54, comprising the primary and secondary mirrors
80 and 82. The azimuth mirror 84 is utilized to redirect the image
forming light bundles into the remainder of the optical system,
namely the spectrum dividing prism and the relay lenses and other
optical components in the optical paths 58 and 60.
Referring now again primarily FIGS. 4, 4A, 4B and 4C, radiation is
reflected off the flat azimuth mirror 84 towards a calcium fluoride
spectrum-dividing prism 56. An image is formed at a Cassegrain
image plane 130 immediately in front of the prism 56. The
spectrum-dividing prism 56 is constructed such that radiation in
the visible and near IR band (about 0.5 to about 0.9 microns)
passes through the prism 56 into the visible/near IR optical path
58 while radiation in the MWIR portion of the spectrum (about 3 to
about 5 microns) is reflected upwards through a fold prism 132,
made from an infrared transmitting material, into the MWIR optical
path 60.
In the visible path, the radiation passes through a relay lens
assembly 62 enclosed in a suitable enclosure 134, a focus element
136 adjusting a set of focus lenses 138, and finally to a shutter
88. An image is formed on the focal plane of the image recording
medium 64. The shutter 88 opens and closes to control exposure of
the visible spectrum image recording medium 64. In the illustrated
embodiment, the medium 64 is a charge-coupled device E-O detector,
comprising an array of pixels arranged in rows and columns. The
array 64 is cooled by a thermo-electric cooler 140. The array and
thermo-electric cooler are enclosed in their own housing 142, which
includes electronics boards 144 and a set of heat dissipating
cooling fans 146.
In the MWIR path, the light passes through a relay lens assembly 66
contained in a suitable housing, through a focus lens assembly 67
and an image is formed at the focal plane of an IR-detecting two
dimensional array 68. The MWIR sensor comprises the array 68, a
cold stop 69, and an internal filter, all enclosed in a cryogenic
dewar 63.
The optical axis of the objective Cassegrain optical subassembly is
shown vertical in FIG. 4. This arrangement provides a very compact
assembly; if the objective were arranged along a horizontal axis
the total length required for the system would have been
intolerably large. The use of only reflecting components
(catoptric) in the objective allows the collection of light from a
very wide spectral region. Such imaging would be impractical or
impossible with a refracting objective design.
The point of intersection of the visible relay optical axis and the
objective optical axis is an important datum feature of this
system. The azimuth mirror 84 reflecting surface is designed to
rotate about an axis that contains this intersection point.
Furthermore, the entire Cassegrain objective subassembly 54 is
arranged to also rotate about this same axis for forward motion
compensation. Rotation of the objective permits locking onto ground
image detail while the camera and aircraft are moving forward. If
the azimuth mirror 84 rotates at half the angular rate of the
objective module with respect to the aircraft/camera frame of
reference, the selected ground image is effectively locked or
frozen onto the detectors. Consequently, the image can be recorded
without blur of relative forward motion between the camera and the
scene.
Presently preferred embodiments of the subject optical system have
focal lengths of between 50 and 100 inches, and an f/number of
between 4.0 and 8.0.
FIGS. 16A and 16B are ray diagrams for the visible and MWIR paths
of the embodiment of FIG. 4. FIG. 16C is a graph of the diffraction
MTF for the visible path. The MTF curves are wavelength-averaged
over the visible/IR spectral range of 500 to 900 nm with system
spectral weights. The Cassegrain objective subsystem introduces a
central obscuration into the light forming beams, and therefore
reduces the diffraction-limited performance limits that can be
achieved.
In the interest of completeness of the disclosure of the best mode
contemplated for practicing the invention, optical prescription,
fabrication and aperture data are set forth below in the following
tables for the embodiment of FIG. 4. Of course, the data set forth
in the tables is by no means limiting of the scope of the
invention, and departure thereof is expected in other embodiments
of the invention. Furthermore, selection and design of the optical
components for any given implementation of the invention is
considered to be a matter within the ability of persons skilled in
the art of optical design of aerial reconnaissance cameras, with
such additional designs being considered obvious modifications of
the illustrated embodiment.
In the tables for the visible and MWIR paths, the numbering of the
elements in the left-hand column corresponds to the optical
elements shown in FIGS. 16A and 16B in progression from the
entrance aperture to the detectors.
TABLE 1 VISIBLE PATH PRECRIPTION FABRICATION DATA 30-Aug-00
Modified 50", F/4 VISIBLE PATH ELEMENT RADIUS OF CURVATURE APERTURE
DIAMETER NUMBER FRONT BACK THICKNESS FRONT BACK GLASS OBJECT INF
INFINITY*1 C-1 11.3468 APERTURE STOP C-2 1 A(1) -11.3468 C-2 REFL 2
A(2) 10.3468 4.8650 REFL DECENTER(1) 3 INF 0.0000 C-3 REFL 3.3525
-8.2000 4 -14.5000 CX INF -0.3000 2.9916 3.0328 CAF2 -0.0200 5 INF
INF -3.1000 3.0382 3.6125 CAF2 -0.4000 6 INF 5.6254 CX .phi.
-0.6360 3.7191 3.7662 'F9474/30' -0.2000 7 INF INF -0.2500 3.6228
3.5785 'OG515' -0.3000 8 -5.4347 CX 33.2980 CX -0.5926 3.4210
3.3057 'A2334/2' -0.0200 9 -4.0545 CX 3.6785 CX -0.8920 3.0291
2.7699 'B2601/1A' 10 3.6785 CC -1.4940 CC -0.2500 2.7699 2.0369
'12549414' -0.4633 11 -2.1854 CX 1.4016 CX -0.8744 1.9875 1.8522
'135662/A' 12 1.4016 CC -0.8475 CC -0.2500 1.8522 1.4085 'B2651/2'
13 -0.8475 CX -1.5553 CC -0.4845 1.4085 1.3037 'A2334/2' -0.3747 14
1.2750 CC -1.5275 CC -0.3634 1.3029 1.6318 'B2650/1' 15 -1.5275 CX
2.1577 CX -0.6838 1.6318 1.7456 '11646656' -0.0270 16 -5.0620 CX
3.3440 CX -0.4730 1.8086 1.8207 'A2334/2' -0.4251*2 17 -7.5223 CX
19.2220 CX -0.1985 1.6606 1.6522 'H9420/8' -0.5725*3 18 -2.5775 CX
-1.1718 CC -0.2500 1.6800 1.5948 'D1741/4' 19 -1.1718 CX 2.7961 CX
-0.7869 1.5948 1.5304 'B2601/1A' 20 2.7961 CC -2.2230 CC -0.2500
1.5304 1.4526 'D1741/4' -1.7614 21 1.4142 CC 21.3497 CX -0.3802
1.7611 2.1914 'H9418/35' -0.0245 22 -3.4440 CX 12.7030 CX -0.8500
2.4681 2.5450 'D1741/6' -0.0500 23 INF INF -0.0800 2.5554 2.5603
BK7 Schott IMAGE DISTANCE = -0.9162 IMAGE INF 2.6440 NOTES Positive
radius indicates the center of curvature is to the right Negative
radius indicates the center of curvature is to the left Dimensions
are given in inches Thickness is axial distance to next surface
Image diameter shown above is a paraxial value, it is not a ray
traced value Other glass suppliers can be used if their materials
are functionally equivalent to the extent needed by the design;
contact the designer for approval of substitutions. APERTURE DATA
DIAMETER DECENTER APERTURE SHAPE X Y X Y ROTATION C-1 CIRCLE 13:168
CIRCLE (OBSC) 4.800 4.800 C-2 CIRCLE (OBSC) 4.800 4.800 CIRCLE
12.600 12.600 C-3 RECTANGLE 3.500 4.000 0.000 0.100 0.0 ASPHERIC
CONSTANTS 2 (CURV)Y 4 6 8 10 Z =
------------------------------+(A)Y + (B)Y + (C)Y + (D)Y 2 2 1/2 1
+ (1 - (1 + K) (CURV) Y ASPHERIC CURV K A B C D A(1) -0.02881267
-1.000000 A(2) -0.05583473 -3.928273 DECENTERING CONSTANTS DECENTER
X Y Z ALPHA BETA GAMMA D(1) 0.0000 0.0000 0.0000 35.0000 0.0000
0.0000 (BEND)
TABLE 2 MWIR PRESCRIPTION FABRICATION DATA Modified 50, F/4 MWIR
LENS 2 Fit to POD Testplates ELEMENT RADIUS OF CURVATURE APERTURE
DIAMETER NUMBER FRONT BACK THICKNESS FRONT BACK GLASS OBJECT INF
INFINITY*1 C-1 11.3468 1 A(1) (Paraboloid) -11.3468 C-2 REFL 2 A(2)
(Ellipsoid) 10.3468 44000 REFL DECENTER(1) 3 INF. -8.2000 3.9482
REPL (Azimuth Mirror) 4 31 14.5000 CX INF -0.3000 3.2080 3.2480
CAF2 (Field Lens) -0.0200 5 INF INF -1.5500 C-3 C-4 CAF2
DECENTER(2) C-4 REFL INF INF INF 1.5500 C-4 C-5 CAF2 0.6250 6 INF
INF 2.4000 C-6 C-7 SILICON DECENTER(3) INF C-7 REFL INF INF -2.5000
C-7 C-8 SILICON 0.1000 7 -5.2363 CX -11 .8133 CC -0.6000 4.3400
4.1700 SILICON -1.2602 8 7.0285 CC A(3) -0.3500 3.1100 3.1800
GERMMW -0.4399 9 3.5768 CC 10.0404 CX -0.3500 3.2000 3.7400 ZNS
-0.0638 10 27.0245 CC 5.1234 CX -0.6000 4.1200 4.2400 SILICON
-1.9286 11 A(4) 10.1487 CX -0.4000 3.8400 3.8200 ZNS -0.9223 12
-2.2473 CX -2.5307 CC -0.3500 2.6700 2.4200 ZNSE -0.2702*2 13
-2.0677 CX -1.4306 -0.5000 2.1000 1.5600 ZNS -0.6180*3 14 INF INF
-0.1180 1.1400 1.1400 SILICON -0.2290 APERTURE STOP C-9 (Cold Stop)
-1.2070 15 INF INF -0.0400 C-10 C-11 SILICON IMAGE DISTANCE =
-2.13000 IMAGE INF 2.8293 NOTES Positive radius indicates the
center of curvature is to the right Negative radius indicates the
center of curvature is to the left Dimensions are given in inches
Thicknes is axial distance to next surface Image diameter shown
above is a paraxial value it is not a ray traced value Other glass
suppliers can be used if their materials are functionally
equivalent to the oxtent needed by the design; contact the designer
for approval of substitutions APPERTURE DATA DIAMETER DECENTER
APERTURE SHAPE X Y X Y ROTATION C-1 CIRCLE 12.633 CIRCLE (OBSC)
4.800 4.800 C-2 CIRCLE 12.500 12.500 CIRCLE (OBSC) 4.800 4.800 C-3
RECTANGLE 2.900 2.900 (CaF2 Prism - Entrance Face) C-4 RECTANGLE
2.900 4.101 (CaF2 Prism - Splitter Face) C-5 RECTANGLE 2.900 2.900
(CaF2 Prism-Exit Face) C-6 RECTANGLE 3.050 3.050 (Silicon
Prism-Entrance Face) C-7 RECTANGLE 3.300 5.640 0.000 -0.149 0.0 C-8
RECTANGLE 3.410 3.410 (Siiicon Prism-Exit Face) C-9 CIRCLE 0.844
0.844 (Cold Stop) CIRCLE (OBSC) 0.320 0.320 (Occulting Disk) C-10
RECTANGLE 1.280 1.280 C-11 RECTANGLE 1.280 1.280 ASPHERIC CONSTANTS
2 (CURV)Y 4 6 8 10 Z = --------------------+ (A) Y + (B)Y + (C)Y +
(D)Y 2 2 1/2 1 + (1 - (1 + K(CURV)Y) ASPHERIC CURV K A D C D A(1)
-0.02381267 -1.000000 A(2) -0.05583473 -3 .928273 A(3) -0.02571678
0.000000 -3.39014E-03 3.98460E-04 -2.25842E-05 0.00000E+00 A(4)
-0.02018930 0.000000 6.40826E-04 8.94387E-05 1.85905E-05
2.93941E-06
Electronics System
The electronics for the camera 36 of FIGS. 1 and 3 is shown in
block diagram form in FIG. 17. The electronics includes an image
processing unit (IPU) 401 which contains the master control
computer 34 of FIG. 1. The master control computer 34 supplies
control signals along a conductor 400 to a camera body and
stabilization electronics module, represented by block 402. The
camera body and stabilization electronics 402 basically includes
digital signal processing cards that provide commands to the roll
motors 70A and 70B and the Cassegrain or azimuth motor 74 of FIGS.
3, 5 and 6, and receive signals from the stabilization system
consisting of the azimuth fiber optic gyroscope 128 mounted on the
azimuth mirror and a roll fiber optic gyroscope (not shown) mounted
on the camera housing 52. The camera body electronics 402 also
receives current roll angle and roll rate data from resolvers in
the roll motors 70A and 70B, and from the roll gyroscope, and
supplies the roll information to the camera control computer.
The camera control computer 34 also generates control signals, such
as start, stop, and counter values, and supplies them via conductor
406 to an IR sensor module (IRSM) 408 and a Visible Sensor Module
410. The IRSM 408 includes a cryogenic dewar or cooler 63, the IR
detector 68 (FIG. 4) and associated readout circuitry, and
electronic circuitry shown in FIG. 19 and described subsequently
for transferring charge through the IR array to achieve roll motion
compensation. Pixel information representing IR imagery is read out
of the array 68, digitized, and sent along a conductor 412 to the
IPU 401. In an alternative embodiment, the electronic circuitry
shown in FIG. 19 could be incorporated into the camera body
electronics 402 or in the Image Processing Unit 401.
The visible sensor module 410 includes a mechanical shutter 88, a
visible spectrum electro-optical detector 64 (FIG. 4) and associate
readout registers, and electronic circuitry described in FIG. 19
and described subsequently for transferring charge through the
visible spectrum detector 64 to achieve roll motion compensation.
Pixel information representing visible spectrum imagery is read out
of the detector 64, digitized, and sent along a conductor 414 to
the IPU 401.
Visible and IR imagery supplied by the Visible Sensor Module and
the IR Sensor Module is received by a dual band input module 420
and supplied to an image processor 422 for purposes of contrast
adjustment, filtering, radiometric correction, etc. Typically,
images generated by the arrays 64 and 68 are either stored for
later retrieval or downlinked to a ground station. In the
illustrated embodiment, the imagery is compressed by a data
compression module 424, supplied to an output formatter 426 and
sent along a conductor 428 to a digital recording module 430 for
recording of the imagery on board the aircraft.
Aircraft inertial navigation system data such as aircraft velocity,
height, aircraft attitude angles, and possibly other information,
is obtained from an aircraft 1553 bus, represented by conductor
432. Operator inputs such as start, stop and roll angle commands
from a manual cockpit or camera console or control panel, can also
be supplied along the conductor 432 or by an optional control
conductor 434. The INS and operator commands are processed in an
INS interface circuit 436 and supplied to the camera control
computer 34 and used in the algorithms described above. The camera
control computer also has a non-volatile memory (not shown) storing
fixed parameters or constants that are used in generating the roll
motion compensation commands, such as the pixel pitch, array size,
master clock rate, and optical system focal length.
The image processor 422 and a graphics module 438 are used to
generate thumbnail imagery and supply the imagery to an RS-170
output 440 for viewing in near real time by the operator or pilot
on board the reconnaissance vehicle, or for downloading to the
ground station. Other format options for the thumbnail imagery are
also possible.
Aircraft power is supplied to a power conversion unit 442, which
filters, converts and distributes it to two power modules 444. The
power modules 444 supply the required AC or DC voltages to the
various electronic components in the camera 36.
An RS-232 diagnostic port 446 is provided in the IPU 401 for remote
provisioning, diagnostics, and software downloads or upgrades or
debugging by a technician. The port 446 provides an interface to
the master control computer 34, and the other modules in the IPU
401 and allows the technician to access these units with a general
purpose computer. Changes to fixed parameters stored in
non-volatile memory, such as a change in the focal length of the
camera, are also made via the port 446.
Except as noted herein and elsewhere in this document, the
individual modules and components in the electronics are considered
to be conventional and therefore can be readily derived be persons
skilled in the art. Accordingly, a detailed discussion of the
modules per se is omitted from the present discussion.
Roll Motion Compensation
Referring now to FIG. 18, a presently preferred implementation of
roll motion compensation in an electro-optical area array detector
will now be described. The visible/near IR E-O detector 64 is shown
in a plan view. The detector consists of an array of pixel elements
300 arranged in a plurality of rows and columns, with the column
direction chosen to be across the line of flight and the row
direction in the direction of flight. The array 64 can be any
suitable imaging detector including a charge-coupled device, and
preferably will comprise at least 5,000 pixels in the row direction
and at least 5,000 pixels in the column direction. The illustrated
embodiment consists of 5040.times.5040 pixels, with a 0.010
mm.times.0.010 mm pixel pitch and a 50.4 mm.times.50.4 mm array
size. The reader is directed to the Lareau et al. U.S. Pat. No.
5,155,597 patent for a suitable detector, however the array need
not be organized into column groups as described in the '597 patent
and could be configured as a single column group, all columns of
pixels clocked at the same rate.
The architecture for the array is not critical, but a full frame
imager, as opposed to an interline transfer architecture, is
presently preferred. The imager can use either a mechanical shutter
or an electronic shutter to expose the array.
The roll motion caused by camera roll motors 70A and 70B produces
an image motion indicated by the arrows 302 in the plane of the
array 64. The roll motion is in the cross-line of flight direction
and the image velocity v is nearly constant throughout the array.
The velocity v is equal to the product of the optical system focal
length f multiplied by the rate of rotation .omega.. Since f is
fixed (and the value stored in memory for the camera control
computer), and the rate of rotation is known by virtue of outputs
of the fiber-optic gyroscope 128 or from resolvers in the roll
motors, the velocity of the image due to roll can be precisely
determined for every exposure. The velocity can be expressed in
terms of mm/sec, in terms of rows of pixels per second, or in terms
of the fraction of a second it takes for a point in the image to
move from one row of pixels to the adjacent row, given the known
pixel pitch. The pixel information (i.e., stored charge) in the
individual pixels 300 is transferred row by row throughout the
entire array 64 at the same rate and in the same direction of image
motion during the exposure time, thereby avoiding image smear due
to the roll motion.
To accomplish this, and with reference to FIG. 19, the camera
electronics includes a counter and clock driver circuit 304 (one
for each detector 64, 68) which generates voltage pulses and
supplies them to a set of three phase conductors 308 which are
coupled to each row of the array. One cycle of three-phase clocking
effectuates a transfer of charge from one row to the adjacent row.
A master clock 306 generates clock signals at a master clock
frequency and supplies them to a counter 310. The camera control
computer calculates a counter value which determines the number the
counter 310 is supposed to count to at the known master clock rate
to time the transfer of charge from one row to another in
synchronism with the movement of the image by one row of pixels
(0.010 mm). The master computer 34 supplies the counter value to
the counter 310, along with a start and stop commands.
At the moment the array 64 is exposed to the scene, the counter 310
starts counting at the clock rate up to the counter value. When the
counter value is reached, a trigger signal is sent to a clock
driver 312. The clock driver 312 initiates one cycle of three phase
clocking on conductors 308, causing the pixel information from row
1 to be transferred to row 2, from row 2 to row 3, etc. When the
counter value is reached, the counter 310 resets itself and
immediately begins counting again to the counter value, another
cycle of clocking is triggered, and the process repeats
continuously while the array is exposed and charge is integrated in
the detectors. At the end of the exposure period, a stop signal is
sent to the counter 310. The pixel information in the array 64 is
read out of the array into read-out registers at the bottom of the
array (not shown), converted into digital form, and either stored
locally on a digital recording medium for later use or transmitted
to a remote location such as a base station.
The process described for array 64 is essentially how the IR
detector operates as well for accomplishing roll motion
compensation. In alternative embodiments, the image motion
compensation could be performed in other readout structures
depending on the architecture for the array. The IR detector could
be sensitive to radiation in the Short Wavelength Infra-Red (SWIR)
band (1.0 to 2.5 microns), Mid-Wavelength IR (MWIR) band (3.0 to
5.0 microns) or Long Wavelength IR (LWIR) band (8.0 to 14.0
microns). In such an array, the output of the each photosensitive
photodiode detector is coupled to a charge storage device, such as
a capacitor or CCD structure, and the charge is shifted from one
charge storage device to the adjacent charge storage device in
synchronism with the image velocity while charge is being
integrated in the charge storage devices.
The process of roll motion compensation can be more finely tuned by
deriving the rate of rotation (.omega.) used in the algorithm from
the actual inertial rate sensed by a fiber optic gyroscope mounted
to the camera housing or frame. Such a gyroscope can count with a
resolution of 1 microradian or better. The gyroscope generates a
signal that is supplied to a DSP card in the camera control
electronics module 402 (FIG. 17). A signal could also be
constructed for imaging array clocking purposes in the form of a
pulse train which the imaging array clock generator could phase
lock to. By doing this, any rate inaccuracy or stabilization
shortcomings associate with the roll motion could be overcome. The
roll motion compensation becomes, in effect, a fine stabilization
system which removes the residual error from the more coarse
electro-mechanical stabilization system. Having a fine system,
based on a closed loop feedback from the roll fiber optic
gyroscope, would allow for a larger range of roll motion without
image degradation.
The above-described roll motion compensation will produce some
minor edge effects at the bottom of the array, which are typically
ignored since they are a very small fraction of the image generated
by the array.
Other Embodiments
As noted above, the principles of roll framing and forward motion
compensation described above are applicable to a camera that images
in a single band of the electromagnetic spectrum. In such an
embodiment, the spectrum dividing prism would not been needed and
the objective optical subassembly (Cassegrain or otherwise) would
direct the radiation in the band of interest to a single optical
path having a photo-sensitive image recording medium placed herein.
The spectrum dividing prism and second optical channel are not
needed. Otherwise, the operation of the camera in roll framing and
spot modes of operation would be the same as described above.
As another alternative embodiment, three or more detectors could
image the three or more bands of the electromagnetic spectrum
simultaneously. In such an embodiment, an additional spectrum
separating prism would be placed in either the visible or IR paths
to further subdivide the incident radiation into the desired bands
and direct such radiation into additional optical paths, each with
its own photo-sensitive image recording medium. As an example, the
visible/near IR band could be divided into a sub 700 nanometer band
and a 700 to 1000 nanometer band, each associated with a distinct
optical path and associated image forming and focusing lenses and
an image recording medium. Meanwhile, the IR portion of the
spectrum could be similarly divided into two separate bands, such
as SWIR, MWIR, and/or LWIR bands, and each band associated with a
distinct optical path and associated image forming and focusing
lenses and an image recording medium. Obviously, in such an
embodiment the arrangement of optical components in the camera
housing will be different from the illustrated embodiment due to
the additional spectrum dividing prisms, additional optical paths
and optical components, and additional detectors. However, persons
skilled in the art will be able to make such a modification from
the illustrated embodiment using routine skill.
As yet an another possible embodiment, the camera may be designed
for hyperspectral imaging. In such an embodiment, one of the
optical paths may be devoted to visible spectrum imaging, while the
other path is fitted with a spectroradiometer, an imaging
spectrometer, or spectrograph to divide the incident radiation into
a large number of sub-bands in the spectrum, such as 50 of such
sub-bands. Each sub-band of radiation in the scene is imaged by the
hyperspectral imaging array.
As yet another alternative, the camera could be mounted transverse
to the roll axis of the aircraft. Such a camera could be used for
dual spectrum, full framing imaging in a forward oblique mode,
either in a spot mode of operation or in a mode in which
overlapping frames of images are generated in a forward oblique
orientation.
As yet another alternative embodiment, the smooth roll motion and
roll motion compensation feature could be adapted to a step framing
camera, such as the KS-127A camera or the step frame camera of the
Lareau et al. U.S. Pat. No. 5,668,593. In this embodiment, the roll
motors are coupled to the step frame scan head assembly, and
continuously rotate the scan head about the roll axis in a smooth,
continuous fashion. The detector array and associated relay and
focusing optical elements remain stationary with respect to the
aircraft. The image acquired by the scan head assembly would need
to be derotated with a pechan prism, K mirror or other suitable
element, as described in the '593 patent. Roll motion compensation
would be performed electronically in the array, as described at
length above.
As a variation on the above embodiment, the roll motors are coupled
to the step frame scan head and continuously rotate the step frame
scan head assembly, while the image derotation is achieved by
rotation of the imaging array in synchronism with the rotation of
the scan head assembly. Roll motion compensation is achieved by
transferring pixel information in the array at substantially the
same rate as the rate of image motion due to scan head
rotation.
Less preferred embodiments of the invention include other types of
optical arrangements. While the catoptric Cassegrain optical system
is the preferred embodiment, refractive optical systems,
catadioptric optical systems, and still other types of optical
arrangements may be used, for example where only single spectrum
imaging is performed, where space requirements are not so
important, or when other considerations dictate that a different
type of optical arrangement for the objective lens is suitable. In
such embodiments, the optical subassembly comprising the objective
lens would be rotated in the direction of flight to accomplish
forward motion compensation as described above, while the entire
camera housing including the objective lens is rotated about an
axis to thereby generate sweeping coverage of the field of
interest, either about the roll axis or about an axis perpendicular
to the roll axis.
Presently preferred and alternative embodiments of the invention
have been described with particularity. Considerable variation from
the disclosed embodiments is possible without departure from the
spirit and scope of the invention. For example, the type and
structure of the image recording medium is not critical. The
details of the optical design, the mechanical system and the
electronics may vary from the illustrated, presently preferred
embodiments. This true scope and spirit is to be determined by the
appended claims, interpreted in light of the foregoing.
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